1 8 HUMAN BIOLOGY 



The amount of snow in the polar caps can be roughly 

 estimated from the fact that the heat which is received 

 fiom the Sun during the season is sufficient to melt and 

 evaporate it, and it is found that when melted they would 

 form a layer of water averaging only a few inches deep. 

 All the water resulting from the melting would be too 

 little to fill Lake Erie, and it is evident that Mars, as a 

 whole, must be a veiy dry planet, and that desert conditions 

 must prevail over most of its surface. 



The spectroscopic test reveals oxygen, too, in the atmos- 

 phere of Mars. The Mt. Wilson observations indicate that 

 the quantity of this gas, above a square mile of the planet's 

 surface, is about 15 per cent as great as for the Earth. 

 The corresponding partial pressure of oxygen at the surface 

 is 6 per cent of the terrestrial value, too little to support 

 human respiration but probably within the limit to which 

 life could adapt itself. The presence of oxygen appears to 

 be intimately connected with a characteristic of the planet, 

 which has been known longer than any other, namely its 

 red color. The greater part of the planet's area is rather 

 uniformly of this hue and all students agree in the belief 

 that in these portions we see the bare surface in its natural 

 color. Now unweathered igneous rocks are not usually red, 

 though they are so occasionally. The incompletely oxidized 

 ferrous compounds give them a grayish or blackish tone, 

 sometimes of a bluish cast. But the fully oxidized products of 

 weathering of such rocks are usually colored yellow or red 

 by ferric oxide. Mars shows just such a color, while among 

 all the other bodies of our system whose bare surfaces 

 we can see (Mercury, the' Moon, and the asteroids and 

 satellites) not one has an atmosphere, nor is one red. The 

 absence of red, even in patches, upon the airless surface 

 of the Moon is especially noteworthy. 



The most interesting of the Martian markings remain 

 to be discussed. The dark areas, which cover about 35 per 

 cent of the surface, are of a greenish or bluish-gray hue, 

 in contrast to the reddish-yellow of the rest. They were 

 once supposed to be seas, but this cannot be true, for, 

 if they were, the reflection of the Sun from the water surface 

 would be by far the most conspicuous feature upon the 



